This application claims the priority of British patent document 0010316.8, filed 28 Apr. 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a demultiplexer control system. In particular, the present invention relates to a control system to enable the control of the demultiplexing process associated with an optical backplane.
The invention has application in the development of new architectures for optical switching applied to high-speed digital communication routers/switches.
UK Patent Application No. 9930163.2—“Data Compression Apparatus and Method Therefor” describes the operation of an optical backplane, for example an optical fiber, in an optical switching system. The operation includes a method for converting packets of data at 10 Gbits/s to packets at 1.28 Tbits/s. In the next step after data compression, the compressed packets are time multiplexed onto the optical backplane of a switching device, for example an IP router or ATM switch. The process of selecting a given compressed packet, from the stream of multiplexed packets on the backplane, is termed demultiplexing the packet.
The operation of an optical switching device wherein pulse trains are time multiplexed on an optical backplane is shown in FIG. 4. A laser generates (step 400) a pulse having a duration corresponding to the length of an input packet of data and a linear chirp, i.e. the frequency of the light increases (or decreases) with time during the pulse. Packets of data corresponding to the input data are received (step 402) by input receiver transducers. The data received is buffered (step 404). The buffered data is then transferred to a modulator controller for modulation (step 406) by a modulator.
A modulated data signal from the modulator is then compressed (step 408) by a compressor. In the case of compressed data pulses corresponding to subsequent input data, a delay is introduced (step 410) to facilitate multiplexing of compressed data pulses. Modulated data signals are combined (step 412) and form a multiplexed compressed modulated pulse train. The combined multiplexed pulse train is carried by an optical backplane then split and sent to at least one modulator where it is demultiplexed (step 414).
The demultiplexed, compressed packet is decompressed once it has passed through the modulator (step 416). The resulting decompressed packet is converted from an optical signal to an electrical signal.
The decompressed, demultiplexed packet is received by output receiver transducers (step 418). The output receiver transducers convert the optical signals received to 10 Gbits/s electrical signals. The signals generated by the output receiver transducers are buffered (step 420) before they are forwarded to output transmitter transducers. Finally, the output transmitter transducers convert the received electrical signal to a 10 Gbits/s optical signal for further transmission (step 422).
Demultiplexing (step 414) is carried out by supplying the multiplexed pulse train of compressed packets to a modulator that is normally in an ‘off’ state. The modulator attenuates, deflects or otherwise blocks the packets whilst in the ‘off’ state. The modulator is arranged to be switched to an ‘on’ state when the required compressed packet arrives, thus allowing the selected compressed packet to pass through the modulator.
In the systems of relevance to IP routers, the modulator is typically in an ‘on’ state for 50 picoseconds (ps). If the time when the modulator is turned ‘on’ does not match the arrival of a compressed packet, part of the compressed packet will be lost and the detected signal will be in error. The timing error has only to be of the order of ten picoseconds to degrade the performance of the switch. Timing errors of this order are common in electrical and optical systems as a result of temperature and other physical changes. In order to ensure that the switch continues to operate effectively, the relative timing of the modulator control signal and the compressed packet must be monitored and periodically adjusted to keep them in their correct relationship.
One approach to achieving this monitoring would be to measure the control pulse and the peak of the compressed data packet and to compare them. This has a number of disadvantages. Firstly, the compressed data packet signal needs to be measured with a very high bandwidth (many tens of gigaHertz (GHz)), which is costly, high in power consumption (therefore heat generating) and difficult to implement.
Secondly, errors on the comparison process are as difficult to avoid as errors in the modulator timing.
Thus there is a need for a technique which enables the timing to be monitored and adjusted without having to measure the signals with picosecond accuracy or bandwidths of many tens of gigaHertz.